The purpose of this proposal is to develop a modified host-mediated assay system by using cultured human lymphoid cells as target cells for screening chemical mutagens and carcinogens. Induction of chromosome damage will be used as an indicator of possible mutagenicity or carcinogenicity of a given compound. First, the growth and cytogenetic characteristics of several selected human lymphoid cell lines will be studied in vitro and after inoculation of such cells in heterologous hosts, second, effects on chromosomes of human lymphoid cells in host by several known mutagens and carcinogens will be tested in this modified host-mediated system and third, the mutagenicity and carcinogenicity of several pesticides in common use will be evaluated by using the modified host-mediated assay developed.